Due to the abundance of natural gas in North America there is a shift occurring from importing Liquefied natural gas (LNG) to exploring the option of exporting LNG. A network of natural gas pipelines exist to supply feed gas to an LNG plant. UOP's SeparSIV™ is an adsorption-based technology as part of an LNG pretreatment step to remove heavy hydrocarbons and prevent freezing of these heavies in LNG production.
The SeparSIV process is based on the principle of temperature swing adsorption (TSA). TSA is a batch-wise process consisting of two basic steps: adsorption and regeneration. In the adsorption step, contaminants or other impurities are removed from the fluid by adsorption onto a solid adsorbent material and then the treated stream leaves the unit with lowered contaminant levels. In the regeneration step, the adsorbed contaminants are desorbed from the solid adsorbent material by means of a regeneration stream (typically a gas stream).
The regeneration step includes two major parts: heating and cooling. In the heating part of the process, the regeneration stream is heated to an elevated temperature and caused to flow over the solid adsorbent material. Due to the heat of the gas and the difference in partial pressure of the contaminants on the solid adsorbent material and in the regeneration gas stream, the contaminants desorb from the solid material and leave the unit with the regeneration gas. A cooler is then used to condense the contaminants from the regeneration gas. In the case that the desorbed contaminants in the regeneration gas cannot be removed by condensation (such as CO2 removal), other separation means are employed to separate the contaminants from the regeneration gas, such as membrane or solvent absorption separation. The cooled regeneration gas, which is saturated with the contaminants, can then be recycled to the feed in a closed-loop mode to minimize the loss of the regeneration gas. Further, because adsorption is promoted by low temperatures, regeneration typically also includes another cooling step subsequent to exposing an adsorbent bed to the hot regeneration gas. In this cooling step, the adsorbent bed is subjected to a flow of cooling gas, which cools the adsorbent bed down from approximately the high temperature of the hot regeneration gas to approximately the low adsorption temperature in readiness for a subsequent adsorption step. Alternatively, in an open-loop regeneration mode, the effluent regeneration gas can be disposed of as a fuel gas or by venting instead of returning to the feed stream.
Hence, the most basic form of a TSA process unit consists of two vessels, with one vessel in adsorption mode and the other vessel in regeneration mode. However, depending on the quantity of feed material to be treated as well as the amount of contaminants to be removed from the feed stream, several vessels, which operate in a parallel mode, or in alternating sequences, could be required. In a more complicated form of operation, the regeneration step can also be split over two vessels in a series-heat-and-cool cycles, where one of the vessels would be in the heating step and another would be in the cooling step.
The pipeline feed gas to an LNG plant may contain oxygen and various sulfur contaminants which can cause operating issues for adsorbent based systems such as SeparSIV. In the various TSA arrangements, the presence of oxygen in the hot regeneration gas can lead to poor performance due to reaction of oxygen with hydrocarbons, resulting in failure to meet the product water specification. Issues with oxygen are amplified in the presence of even ppmv levels of sulfur, such as H2S or mercaptans, due to formation of elemental sulfur in the bed which can substantially impact bed life and overall unit performance. These reactions can occur during the high temperature bed regeneration step. In general, with respect to the downstream LNG requirement, oxygen and sulfur compounds can be slipped into the product stream without being removed by SeparSIV. There remains a need for a process solution to address the presence of oxygen and sulfur impurities.
Accordingly, it is desirable to provide TSA systems and associated fluid purification methods that address the presence of oxygen and sulfur compounds present in the feed to the LNG plant. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.